Enhancement of the catalytic performance of Co-ZIF/WO3 heterostructures for selective catalytic reduction of NOx

Nitrogen oxides (NOx) are one of the growing air pollutants in industrial countries, and their emissions are regulated by stringent legislation. Therefore, the design of the catalyst comprised of metal oxides and ZIFs a potential solution for improving selective catalytic reduction (SCR) of NOx. Here, an efficient strategy was described to fabricate Co-ZIF/WO3 heterostructures for SCR of NOx. First, WO3 nanostructures were fabricated by the solvothermal method, and subsequently epitaxial growth of ZIF-67 on the metal oxide surface to create a new type of semiconductor Co-ZIF/WO3 heterostructures. The obtained heterostructures were systemically characterized by wide-angle XRD, FESEM, UV DRS, FT-IR, AFM, and TEM spectroscopies. The Co-ZIF/WO3 heterostructures shift the temperature corresponding to the maximum conversion around 50 °C towards lower temperatures. The maximum conversion is substantially enhanced from 55% at 400 °C to 78% at 350 °C. The enhanced activity is attributed to better interaction and synergic effect of WO3 incorporated into ZIF-67 and also the electron transfer facility between the WO3 and Co species in Co-ZIF/WO3 heterostructures. Moreover, Co-ZIF/WO3 results in a distinct effect on the production of carbon monoxide (CO) in the product gas stream. The current study highlights some of the challenges in the development of semiconductor-based heterostructures for a decrease in air pollution.

investigate have reported the efficient method of selective catalytic reduction (SCR) for NOx abatement via different hydrocarbon compounds such as propane 19,20 , methane 21 , and ammoniac as a reductant.Moreover, semiconductor materials with different charge migration pathways have been used in an industrial SCR system 22 .WO 3 -based materials as efficient semiconductors have been considered to be a promising photocatalyst in abating NOx emissions from air pollution 23 .Unfortunately, the low electrical conductivity and negligible specific surface area have restricted the performance of pure WO 3 in the SCR-NOx-Propane process.Fortunately, the conjugation of this semiconductor material with Zeolites could improve light harvesting and charge separation for the removal of NOx 24 .Zeolites such as ZSM-5 25 , clinoptilolite 26 , and metal oxides including cerium oxide 27 , zirconium oxide 28 , vanadium oxide 29 , and tungsten oxide 23 have been proposed as catalysts for the removal of NO x with various methodologies.Recently, Lee et al. developed CuSn/ZSM-5 for the HC-SCR-NOx process 30 .Moreover, the effect of the addition of molybdenum on the enhanced low-temperature SCR of NOx by NH 3 over MnOx/γ-Al 2 O 3 catalysts was investigated by Yang and et al. 31 .Furthermore, Zhan et al. reported mesoporous WO 3 for SCR-NOx with NH 3 32 .Recently, our research team developed engineered nanostructures to expand the photo-based advanced oxidation process (AOP) [33][34][35][36] and persulfate-based AOP 4,[37][38][39] for the removal of organic contaminations from wastewater.
Zeolitic imidazolate frameworks (ZIFs) with the advantages of uniform pore size and high BET surface have been used as catalysts for selective catalytic reductions (SCR) 40,41 .Recently, ZIF-based materials like ZIF-67 have been utilized as an efficient microporous for NO x abatement and its performance in the SCR-NOx process has been investigated 42 .Recently, Zhao et al. reported Cu-ZIF performance on NH 3 -SCR-NOx 42 .Moreover, the performance of Co 3 O 4 -PC derived from ZIF-67 for low-temperature SCR of NOx by ammonia was studied by Bai et al. 42 .However, the activity of ZIF-67 on propane-SCR-NOx has not been studied so far.Therefore, the activity of ZIF 67 as an efficient catalyst for NOx abetment is investigated.In this study, we have attempted to develop an efficient nanostructure for promoting the activity of WO 3 as a conventional catalyst for NOx abatement.Accordingly, a porous nanocomposite of Co-ZIF/WO 3 due to the better interaction and synergic effect of WO 3 nanostructure incorporated into ZIF-67 was used.Also, the electron transfer facility between the WO 3 and Co species in the channels of Co-ZIF/WO 3 exhibited a lower energy band gap of Co-ZIF/WO 3 led to enhancing the catalytic activity of Co-ZIF/WO 3 in the NOx process.Therefore, a part of the study belongs to the design, fabrication, and characterization of nanocomposite of Co-ZIF/WO 3 heterostructure and another part belongs to the performance of WO 3 , ZIF-67, and Co-ZIF/WO 3 in conversion and temperature on propane-SCR-NOx.According to the latest findings reported, there is no study on the design of Co-ZIF/WO 3 nanocomposite for selective catalytic reduction (SCR) of NOx.Therefore, the current study provided a rational design for a decrease in air pollution.

Characterization
The characterization of as-prepared nanostructures and catalytic reduction of NO x is described in text S1 of supplementry Information.

Fabrication of WO 3 nanoplates
The WO 3 nanostructure was prepared following a method described by Zheng and co-workers 43 .In a typical experiment, 8.25 g Na 2 WO 4 •2H 2 O was dissolved in 25 mL dionized water under sonication.Subsequently, 2.0 mL of 2.0 M HCl was added to the mixture reaction and then oxalic acid was added to the mixture to adjust the final pH value to approximately 2.50 and diluted to 250 mL.After that, 1.17 g NaCl was added to the mixture reaction under sonication.After 10 min sonication, 70 mL of the above precursor solution was transferred into a 100 mL of Teflon-lined autoclave and heated for 4.0 h at 170 °C.After completion of the reaction, the resulting yellow precipitate was washed several times with DI-H 2 O and EtOH for purification to obtain the final WO 3 nanoplates.

Fabrication of Co-ZIF/WO 3 heterostructure
To fabricate Co-ZIF/WO 3 heterostructures, firstly 0.20 g of as-synthesized WO 3 with 2.0 mmol PVP as structuredirector agents was dispersed in 100 mL MeOH under sonication.Subsequently, 1.05 g of Co(NO 3 ) 2 •6H 2 O was added to the mixture reaction shaken vigorously for 15 min.Afterward, 6.15 g 2-MeIM was added to the mixture reaction under stirring for 24 h to grow ZIF-67 microcrystals on the WO 3 surface.Finally, the prepared precipitates were separated by centrifugation and washed with MeOH several times, and dried at 80 °C under a vacuum.The schematic fabrication of the Co-ZIF/WO 3 heterostructure is revealed in Fig. 1.

Investigation of catalytic activity
The schematic catalytic reactor system used in this study was displayed in Fig. 2. In brief, the inlet gas mixture involved nitric oxide (30 mg/L), nitrogen dioxide (460 mg/L), oxygen (2.5 vol%), C 3 H 8 (1.0 g/L), and also argon gas (balance) was introduced to the flow meter set at 300 mL/min.Afterward, the gas mixture was preheated and also conducted in an integral reactor containing 0.50 g of the catalyst.A stainless steel vessel was used as a reactor with a diameter of 0.50 inches.An electrical furnace was applied to heat the reactor.Various thermocouples at the inlet, inside the reactor bed, and outlet of the stream were employed to control system temperature.This system controlled the temperature of the reactor bed over 150-400 ± 1 °C.Gas analysis of the outlet was measured using a sensor probe.Finally, the KANE 940 gas analyzer was conducted to evaluate gases of NO, NO 2 , O 2 , and CO.

Result and discussion
The crystalline phase of WO 3 and Co-ZIF/WO 3 was assessed using the HA-XRD pattern.As depicted in Fig. 3a, the characteristic diffraction peaks at 22.1°, 23.6°, 24.3°, 34.1°, 41.7°, 49.8°, 54.9°, and 55.8° be indexed α-WO 3 with monoclinic phase and JCPDS Card No. 83-0950 36 .After epitaxial growth of ZIF-67 on surface WO 3 , all peaks related to crystal ZIF-67 were revealed in ZIF-67/WO 3 heterostructure 38,39 .Moreover, the chemical groups of the heterostructure were evaluated by FT-IR spectra (Fig. 3b).The peaks at 600-800 cm −1 can be ascribed to W-O-W stretching vibration 44 , confirming the formation of WO 3 .The weak peaks at 460 cm −1 revealed the vibration of the Co-N, respectively [45][46][47] and bands in the range of 800-1300 corresponded to the symmetric and asymmetric stretching of the imidazole rings 47 , confirming the formation of ZIF-67 on WO 3 surface.The optical response characteristics of as-obtained nanostructures were evaluated by UV-Vis DRS spectra.As shown in Fig. 3c, the absorption edge of WO 3 was approximately 470 nm, while after epitaxial growth of ZIF-67 on WO 3 exhibited a strong absorption intensity from 350 to 650 nm.Meanwhile, the broad absorption in the UV region (< 400 nm) can be attributed to ligand to metal charge-transfer (LMCT) 36,48 , while displaying three absorption peaks at 538, 565, and 590 nm indexed to the 4 A 2 (F) → 4 T 1 (P) transition of Co 2+ ions in ZIF-67 framework [48][49][50] .Also, the band-gap values for WO 3 and ZIF-67/WO 3 heterostructure were 2.71 and 1.89 eV, respectively (Fig. 3d), indicating an increase in light absorption capacity.As depicted in Fig. 3e, Co-ZIF and WO 3 revealed positive slopes in the linear regions of the Mott-Schottky plots, illustrating both the nanostructures have n-type semiconductor behavior.Meanwhile, the flat band potential (E fb ) derived from Mott-Schottky plots of WO 3 and Co-ZIF were approximately − 0.19 and − 0.37 V (vs Ag/AgCl, pH 7), which are equivalent to − 0.04 V and − 0.23 (vs NHE, pH 0) Eq. ( 1) 36 .According to (Eqs.( 1)-( 3) 36 and Mott-Schottky plot, the conduction band (CB) and valence band (VB) of WO 3 were calculated to be − 0.04 eV and 2.67 eV, while those of Co-ZIF were obtained to be − 0.23 eV and 1.66 eV, respectively (Fig. 3f).
(1) E (Vs.NHE, pH 0) = E (Vs.Ag/AgCl, pH 7) − 0.0591 7− pH electrolyte + 0.198  www.nature.com/scientificreports/Moreover, N 2 adsorption-desorption isotherm was conducted for the evaluation of the surface area and pore size of Co-ZIF/WO 3 heterostructure.As depicted in Fig. 4a and Table 1, the heterostructure indicated a typical type I isotherm with surface area S BET = 1061 m 2 /g and total pore = 0.35 cm 3 /g.Meanwhile, ZIF-67 and WO 3 revealed a typical type II and III isotherms with S BET = 1420 m 2 /g and 16.17 m 2 /g, respectively.According to the BJH plot (Fig. 4b), the corresponding pores diameter distributions of the Co-ZIF/WO 3 , ZIF-67, and WO 3 were specified at 1.21, 1.23, and 4.69 nm respectively.These microporous structures with larger specific surface areas in Co-ZIF/WO 3 heterostructure can be provided suitable to the adsorption of gases and catalytic activity.
The morphology and structure of the prepared catalysts were evaluated by FESEM, AFM, and TEM.The pure WO 3 exhibited a plate-like structure with a thickness of about 45 nm (Fig. 5a).After the growth of the ZIF-67 on WO 3 , these metal oxides were heterogeneously dispersed over the surface of ZIF-67 (Fig. 5b).Meanwhile, the structures and morphology of ZIF-67 and WO 3 about preserved after composition.Also, SEM-mapping (Fig. 5c) and EDS analysis (Fig. S1, ESI) demonstrated the purity and elements corresponding to the heterostructure.Moreover, the presence of WO 3 nanoplates in the core of the proposed heterostructure was not observed with www.nature.com/scientificreports/FESEM images; therefore, the TEM image was conducted to further evaluated the morphology and growth of ZIF-67 with WO 3 .As displayed in Fig. 5d, WO 3 nanoplates were observed on the surface of ZIF-67 with rhombic dodecahedral framework morphology.This image further confirmed the formation of Co-ZIF/WO 3 to composite form.Furthermore, AFM analysis indicated surface morphology and roughness of the proposed heterostructure.
As depicted in Fig. 5e and f, the arithmetic average roughness (Ra) of Co-ZIF/WO 3 was approximately 150 nm, which appeared in light and dark regions in the 2D and 3D images.
After characterization of the proposed heterostructure, the activity of WO 3 , ZIF-67, and Co-ZIF/WO 3 was evaluated on the conversion of NOx to N 2 .The total NOx to N 2 conversion curve as a function of reaction temperature for all samples at gas hourly space velocity (GHSV) 51,048 h −1 .As depicted in Fig. 6a, all samples reveal a similar trend in NOx reduction efficiency.Meanwhile, all samples have a maximum conversion at 350-400 °C, then activity was decreased.The smallest conversion factor (55%) belongs to WO 3 nanoplates at 400 °C due to not being porous material and having a low specific surface area.Also, the maximum activity occurred for ZIF-67 and Co-ZIF/WO 3 heterostructure with a conversion rate of 62% and 78% at 350 °C, respectively.Therefore, the corresponding maximum transition temperature changes from 50 °C to a lower temperature for ZIF-67 and Co-ZIF/WO 3 heterostructure.Research has revealed that WO 3 is an n-type semiconductor with negligible porous structure, which has the least conversion of NOx into N 2 in comparison to ZIF-67 and Co-ZIF/WO 3 .Meanwhile, the ZIF-67 indicated a suitable conversion of NOx into N 2 due to the microporous structure and high surface area (S BET = 1420.34m 2 /g, Table 1 and Fig. S2, ESI).Also, ZIF-67 contains cobalt species that can promote the active site in the SCR-NOx process.Therefore, the composition of WO 3 nanoplates with ZIF-67 microcrystals led to increasing catalytic activity due to interaction and electron transfer facility between the WO 3 and Co species in the Co-ZIF/WO 3 .It should be noted that in some additional runs, the mass of prepared nanostructures and GHSV were increased by 20% at the constant reaction temperature.Since no significant changes in turnover were observed at the concentrations of N 2 , NO 2 , NO, and CO.It was easy to conclude that there was no resistance to mass transport of the gas layer in any of the other experiments.Figure 6b demonstrates the development of the NO 2 concentration in the flue gases as a function of the reaction temperature in the range from 150 to 500 °C for different samples at GHSV 51,048 h −1 .An S-shaped curve (decreasing with increasing temperature) was  obtained for each sample.Particularly noteworthy is the variability of the NO concentration in the generated gas depending on the reaction conditions.Also, the change in the NO concentration of the generated gas with the reaction conditions is worth special evaluation.It is known that Mo (650 °C), Ag (160 °C), and stainless steel (450 °C) can catalyze the decomposition reaction of NO 2 to NO.As depicted in Fig. 6b and c, Co-ZIF/WO 3 revealed the lowest NO production concentration and the highest NO 2 concentration reduction.It should be borne in mind that the main nitrogenous reagent in our system is NO 2 due to the decomposition of NO 2 into NO and O 2 at temperatures above 200 °C.As depicted in Fig. 6d, in a three-component system NO 2 -NO-O 2 conversions up to 50% are expected at 500 °C due to the residence time of NO 2 species in the reaction system.The findings indicated the formation of CO as an undesired product is inevitable during the NOx reduction in presence of hydrocarbons.As displayed in Fig. 6e, the CO production concentration in Co-ZIF/WO 3 heterostructure is a minimal value compared to WO 3 nanoplates and ZIF-67 microporous (35 ppm at 500 °C).According to our previous studies 26,51 , this phenomenon may be related to the non-selective combustion of hydrocarbons (propane) at higher temperatures.Furthermore, the CO generation reaction can activate in Co-ZIF/WO 3 heterostructure channel and does not exclusively take place in the gas phase.However, it is observed that the CO concentration increases with the increase of the reaction temperature, which can be justified by considering the following reactions [Eqs.( 4)-( 8)] 26 . (

Conclusion
The present study was planned to design and fabricate Co-ZIF/WO 3 heterostructure for NOx reduction emissions.The results obtained from the XRD, FESEM, and TEM indicate the epitaxial growth of ZIF-67 microcrystals onto the WO 3 nanoplates.The catalytic efficiency of the Co-ZIF/WO 3 in terms of NOx reduction is better than that of Co-ZIF, and WO 3 .This enhanced activity is attributed to the synergic effect of WO 3 nanoplates loaded into ZIF-67 microcrystals and also the electron transfer facility between them in Co-ZIF/WO3 heterostructures.Moreover, the outlet concentration of CO is lower for Co-ZIF/WO 3 than for ZIF-67, and WO 3 , which is an undesirable product in the SCR-NOx-Propane process.Our future efforts will be devoted to designing a new type of nanostructure for the reduction of the CO concentration to near 5 ppm.Finally, our findings provide a feasible strategy for the development of semiconductor-based heterostructures for reducing NOx emissions.

Figure 2 .
Figure 2. Catalytic reactor for the SCR-NO x process in the study.

Figure 6 .
Figure 6.The effect of the reaction temperature and kind of sample on the conversion of NO x (NO + NO 2 ) into N 2 (a), the variation of NO concentration (b), the variation of NO 2 concentration in the exhaust gas for different nanostructures (c), The equilibrium conversion of NO 2 to NO in the three component system NO 2 -NO-O 2 (d), and outlet CO concentration of production versus reaction temperature for all samples (e).

Table 1 .
Textural properties of samples.